1. Field of the Invention
This invention relates to compositions and the methods for using them for inhibiting corrosion of metal due to chlorides.
2. History of the Related Art
There has been a great need for a chemical which will, in the presence of chlorides, inhibit the corrosion of steel; and considerable research has gone into seeking such a chemical.
Steel is exposed to the chloride ion from several sources. Structures which are exposed to the environment of ocean air and water are seriously corroded by the chlorides present. When calcium chloride is used as a heat exchange solution in refrigeration plants, corrosion inhibitors are added to inhibit the corroding effect of the chlorides. In order to accelerate the setting of concrete, calcium chloride has for years been added to the concrete and sad experience has taught that the contained chlorides will corrode the reinforcing steel in the concrete. Probably the most perplexing source of chloride corrosion has come from deicing the paved surfaces of the roads and highways. The chloride solutions thus produced are splashed onto the steel in the highway structures as well as onto the vehicles which travel the roads. The corrosion thus produced is quite visible on rusted structures and vehicle bodies. Also visible are the pot holes which are produced as the corrosion, produced by chlorides on reinforcing steel, expands, puts a stress on the concrete and breaks out large pieces of concrete. Repairing this damage and that to ocean front buildings is a major expense.
As an illustration of the research which has been done to inhibit the corrosion of steel and to protect the steel from the effects of chlorides, refer to "Corrosion of Reinforcing Bars In Concrete" by Mozer, Bianchini and Kesler, from the JOURNAL OF THE AMERICAN CONCRETE INSTITUTE, August 1965. The PORTLAND CEMENT ASSOCIATION, in 1968 put out a publication titled, "Concrete Information". Also in 1968, the PORTLAND CEMENT ASSOCIATION put out a publication "Corrosion of Steel in Concrete" by John G. Hendrickson. The AMERICAN CONCRETE INSTITUTE put out a publication titled, "Corrosion of Reinforcing Steel", by Bailey Tremper. In addition, these publications list dozens of additional publications on the same subject.
Many attempts have been made to find inhibitors which would prevent the corrosion of steel. Gouda and Monfore, in a publication titled, "A Rapid Method for Studying Corrosion Inhibition of Steel in Concrete" discuss trying some of the well-known corrosion inhibitors, such as sodium nitrite, potassium chromate and sodium benzoate. In order to inhibit the corrosion effects of a 2% addition of calcium chloride to concrete, they found that it requires the addition of 1% to 2% of sodium nitrite and 2% to 4% of potassium chromate. Because sodium benzoate is precipitated by calcium chloride they tested sodium benzoate against 1.6% of sodium chloride and found that it took 4% to 6% of sodium benzoate to inhibit the corrosion from this amount of sodium chloride.
Because of their side effects, most inhibitors involve problems. As an example, the chromates, and there are other effective chromate salts, are highly toxic. If they contact the skin they may produce ulcers. In a Portland Cement Association publication of 1968 CONCRETE INFORMATION, "Effect of Various Substances on Concrete and Protective Treatments, Where Required", indicates the publication that sodium nitrite will cause slow disintegration of concrete. To date, a satisfactory corrosion inhibitor for chlorides has not been found.
The above inhibitors are anodic inhibitors. They can form, according to Gouda and Monfore, "sparingly soluble iron salts or gamma Fe.sub.2 O.sub.3 films on the anodic areas, thus preventing ferrous ion from passing through into solution." These salts are formed, in conjunction with the classic understanding, where the corrosion of steel is considered to be electrochemical. At the anode, the iron goes into solution, forms a ferrous ion and releases two electrons. Fe.fwdarw.Fe+++2e-. At the cathode the two electrons react with two hydrogen ions 2H.sup.+ +2e.sup.+ .fwdarw.H.sub.2 to form a hydrogen film. When the supply of oxygen is limited and the pH is relatively high, an anodic film builds up on the steel to stop the corrosion. When chloride ions are present, the protective anodic films are removed by forming soluble chloride compounds and the steel is left open to further electrochemical attack.